Multiple screw driving machines are previously well known in the art and it is also well known to arrange the individual screwdrivers included in the multiple screw driving machine to be separately drivable independently of each other. This is desirable in order to be able, for instance, to tighten the screws in a screw joint in a certain order. Such a multiple screw driving machine is fairly easy to accomplish by providing each individual screwdriver with a separate motor which can be, for instance, electrically, pneumatically or hydraulically driven. As a result, it is possible to tighten and loosen the screws in the desired order by successively driving the motors associated with the respective screwdrivers. Such a multiple screw driving machine is known, for instance, from DE 2 112 907.
However, in order to reduce the cost of the multiple screw driving machine and also the space required for each individual screwdriver it is also desirable to provide driving by means of one common motor. By optional connection and disconnection of the driving connection of the respective screwdrivers, it is possible to drive an arbitrary number of the screwdrivers in an arbitrary order.
SE 456,072 discloses a multiple screw driving machine of the kind mentioned by way of introduction. It has one common motor which drives a centrally mounted driving shaft. The individual screwdrivers are distributed around the driving shaft, and the drive connection or drive transmission between the driving shaft and the individual screwdrivers is provided by a gear wheel which is arranged for each screwdriver and which is, on the one hand, rotatably arranged on the shaft of the screwdriver and, on the other, displaceable in the longitudinal direction along the same. The gear wheel is continuously engaged with a non-rotatable gear rim arranged on the driving shaft, and thus the gear wheel of all the screwdrivers will rotate as soon as the driving shaft rotates. At one end of the respective gear wheels, there are engagement means which can engage mating engagement means which are arranged on a circumferential flange on the screwdriver shaft when the gear wheel is moved to a front end position, and then the screwdriver shaft is drivingly connected with the driving shaft. On the other hand, when the gear wheel is moved to a rear end position the engagement means are not engaged with each other and the screwdriver shaft is not drivingly connected with the driving shaft but is idle even if the driving shaft and consequently also the gear wheel rotate. The operation of the gear wheel between the rear and the front end positions is ensured with the aid of a pneumatic operating means which is arranged parallel to and on the outside of the respective screwdrivers and which engages the gear wheel by means of a fork-shaped element in a circumferential groove in the gear wheel. This multiple screw driving machine suffers from a number of drawbacks. It requires, among other things, an expensive and complicated construction for the gear wheel to be rotatable about the respective screwdriver shafts and displaceable in the longitudinal direction in relation to the same. Furthermore, the multiple screw driving machine is limited by the individual screwdrivers being distributed about the driving shaft to allow the respective gear wheels to directly engage the gear rim on the driving shaft. Thus it would not be possible to tighten a screw joint in which the screws are placed, for instance, in a straight line. In addition, the operating means is arranged on the side of the respective screwdrivers and parallel to their respective shafts, which requires a great deal of space and may constitute an obstacle when the available space is small.